MAGNETIC STIMULATION OF EXCITABLE TISSUE - CALCULATION OF INDUCED EDDY-CURRENTS WITH A 3-DIMENSIONAL FINITE-ELEMENT MODEL

Magnetic stimulation is the process of inducing eddy-currents in excitable tissue by a time varying magnetic field. Time varying gradient fields in high speed Magnetic Resonance Imaging (MRI) may be of sufficient intensity to stimulate the patient. In this work, the eddy-current distribution in a 3D inhomogeneous model of the dog's thorax due to time varying fields in MRI x, y and z-gradient coils and pair of coplanar coils are calculated. From experimental data of cardiac threshold from a z-gradient coil for a 540 mu s rectangular pulse, a current density of 21.5 A/m(2) was calculated. For a co-planar coil pair, the threshold current density for cardiac stimulation is 19.2 A/m(2) at duration 640 mu s The different conductivities of tissues in the thorax have a significant influence on the eddy-current flow. Calculations combined with measurements will help identify, safe levels of pulsed gradient fields in future MRI systems.